The formation of scale inside of pipes that transport water and other fluids is both a common and serious problem. This holds true for industrial as well as residential applications. The reason is that most fresh water in the United States can be regarded as "hard." Hard water contains mineral ions, such as calcium and magnesium, which are dissolved in the water but precipitate out over time onto the interior surfaces of pipes and other conduits through which the water travels.
More particularly, in scale formation, supersaturated mineral ions (such as calcium) in the water combine with counter-ions (such as bicarbonate) and then precipitate out of solution and deposit on scale-susceptible surfaces.
As scale deposits on the inside surface of the pipe accumulate, the effective diameter of the pipe is reduced, thereby restricting the flow of water. (In a vessel for holding water, scale reduces the vessel's capacity.) This is an especially acute problem when the water is used in a heat transfer situation such as a boiler. Since the scale deposits act as a heat insulator, it contributes to the further deleterious effect of reducing the efficiency of the heat transfer. This reduction can be very expensive, causing as much as 70% of the overall cost of the heating fuel to be wasted.
One way of dealing with scale deposits is to physically remove the deposits by such procedures as sand blasting, acid cleaning, mechanical scraping or brush punching. However, these procedures generally require at least some disassembly of the equipment in which the scale deposits have formed, with consequent interruption in the operation of that equipment, in addition to the cost of the procedures themselves. Moreover, these physical methods of removing the scale may damage the pipe or other pieces of equipment. Furthermore, certain sections of pipe may be inaccessible to a scraper or brush, making it impossible to remove the scale with a physical procedure.
Procedures using physically non-invasive steps are known in the art. For example, it is known to wrap a wire in an elongated, spiral configuration around a segment of the pipe, upstream from the location where the scale deposits would normally form. The spirally-wrapped wire forms an induction coil. A time-varying electric current is passed through the coil, thereby creating a time-varying magnetic field inside the segment of pipe around which the wire is wrapped. That magnetic field produces induction and this induction in turn causes the mineral ions to precipitate out of the water. This effect by an electronic descaling apparatus is called "controlled precipitation".
Calcium carbonate precipitates are capable of assuming two predominant crystal structures. One of these crystal structures floats in solution and can be carried away by the water flow; the other crystal structure tends to cling to the lateral pipe surfaces and/or to sink to the bottom and hence accumulates to form the undesired deposits.
It is a goal in controlled precipitation to promote the first crystal structure which floats in the water. This is achieved by forming, upstream of the region of the potential scale deposits, seed crystals of the above-described structure which float in solution. These seed crystals cause subsequent mineral carbonate precipitates of the same structure to grow around them. These precipitates then pass through the (downstream) region of potential scale deposits without causing the (undesired) formation of such deposits.
As previously noted, a known technique for effecting controlled precipitation involves spirally wrapping an induction coil around the above-mentioned upstream portion of the pipe and energizing this coil with a time-varying electric current. This current in the induction coil creates a time-varying magnetic field inside the pipe and that field, in turn, causes induction which then in turn produces the desired controlled precipitation.
This known technique is relatively effective in dealing with undesired scale formation, but there are situations in which its implementation is problematic. For example, there are installations in which the diameter of the piping around which the induction coil would need to be wound is so large (as much as 84 inches in some electric power generating plants) that it would be impractical to wrap the required induction coil around the perimeter of the pipe due to its physical size and the large impedance created by the long length of wire needed to form a large diameter coil.
As another example, there are installations in which the piping is in such tight quarters, or so close to other equipment, that it would again be impractical to wrap the induction coil around the outer perimeter. Also, the larger the diameter of the pipe, the greater the electric current that is needed to effect the precipitation of the calcium carbonate.